Insecticide use for soybean aphid control up again in 2005

by Matt O'Neal, Department of Entomology

The summer of 2005 saw an increase in soybean aphid populations from very low populations experienced in 2004. Although not completely unexpected, the differences between 2004 and 2005 are still remarkable. Last year, many field crop entomologists were at a loss for data, with soybean aphid populations peaking at no more than a couple hundred per plant. This year we collected plenty of data, with long days spent counting sometimes thousands per plant. The record for aphid outbreaks was set in Michigan. Chris DiFonzo, field crop entomologist at Michigan State University, reported nearly 90 percent of the soybeans planted in the state were treated for soybean aphids. In the Lower Peninsula, populations went well above our current recommended threshold (250 per plant) in June and reports of spraying continued into August.

Soybean aphids will lay eggs near bud scales on limbs of buckthorn in the fall and then they overwinter there. (David Voegtlin)

At a September meeting, I conducted a brief survey of the Iowa field crop specialists to get a sense of how many acres of soybeans were treated for soybean aphid in Iowa. I asked each of the 12 agents to estimate the acreage that was treated with a foliar insecticide. If they gave a percent, I estimated the acreage based on 2004 harvested acreage estimates from the National Agriculture Statistics Services (www.usda.gov/nass/). This is an underestimate of total insecticide usage in soybeans as spider mite outbreaks occurred that are not part of this estimate. Also, I did not ask the specialists what percentage had an insecticide applied directly to the seed. The goal is to provide a general sense of the intensity and location of insecticide usage in Iowa soybeans. I will follow up with more direct estimates during the fall and winter months.

Of the 12 field crop specialists, all but one provided an estimate. Todd Vagts of Region 6 (Figure 1) is conducting his own survey and has not yet received responses from it. In total, we estimated a little over 2.1 million acres treated (Table 1). The greatest amount of insecticide was used in the northern third of the state (top 4 regions = 2, 4, 1, 5). Although the actual amount may vary from this estimate, it is interesting to note that last year, based on a similar survey, I estimated only 100,000 acres treated. It appears that the year-to-year fluctuations of this pest have continued. It will be very interesting to determine how this pattern plays out across the Midwest. Are regions of Iowa (and the Midwest) closest to large amounts of the overwintering host (buckthorn) more at risk for aphid outbreaks?

Table 1. Estimates of acreage treated with a foliar insecticide to manage the soybean aphid during the 2005 growing season by regions (see Figure 1).

1.

443,840

7.

51,312

2.

658,080

8.

8,000

3.

110,000

9.

150,000

4.

512,575

10.

1,000

5.

152,000

11.

15,000

6.

NA

12.

43,240

The suction trap network that was discussed last issue attempts to collect aphids as they conduct migratory flights between summer and fall hosts. In the past four years, we have used data collected from suction traps deployed only in Illinois to predict the overwintering success of soybean aphids. This year we have expanded that network to include several states, including Iowa. Yet there are still many questions that need to be answered. At what spatial scale is the risk of spring aphid immigration suggested by suction trap data realized? What other factors (soil fertility, multiple pest interactions, predation from insect predators, weather) contribute to such a prediction? We will continue to provide updates with regards to both our suction trap network and insights into soybean aphid ecology and pest management.

This article originally appeared on pages 199-200 of the IC-494(24) -- October 10, 2005 issue.